Modern integrated circuits (“ICs”) are designed to perform multiple functions. For example, many ICs include logic circuits as well as memory elements to enable the ICs to perform logic functions and store data. One conventional manufacturing process for such ICs provides multiple gate oxide thicknesses in a single oxide layer. A “dual-gate-oxide-thickness” process is one conventional manufacturing process that forms a single oxide layer with two varying thicknesses. Generally, the dual-gate-oxide-thickness process involves two oxidation steps with a masked etching step between the two oxidation steps to decrease the thickness of certain regions of the oxide layer. However, as the devices continue to be scaled to sub-micron dimensions, the etching process becomes problematic and a strong gate oxide layer becomes increasingly difficult to obtain.
Another conventional approach to providing dual gate oxide thicknesses is through nitrogen implantation because the oxidation rate may be slowed when nitrogen is present in silicon layer upon which the oxidation process takes place. Therefore, selective implantation of nitrogen will allow the formation of thicker oxide portions in non-implanted silicon regions and thinner oxide portions in nitrogen implanted silicon regions. However, nitrogen implantation sometimes creates implantation-induced defects that would be inevitably incorporated into the oxide layer through the oxidation process. These defects may degrade the gate oxide integrity. In addition, the implanted nitrogen might be unintentionally diffused further into the silicon layer during the high-temperature oxidation process. As a result, oxide thickness and uniformity become difficult to predict or control.